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SSN-774 Virginia-class
NSSN New Attack Submarine
Centurion
The Secretary of Defense in his October 1993 bottom-up review determined that production of the Seawolf class submarine would cease with the third submarine, and that the Navy should develop and build a new attack submarine as a more cost-effective follow-on to the Seawolf class, with construction beginning in fiscal year 1998 or 1999 at Electric Boat. The New Attack Submarine is the first U.S. submarine to be designed for battlespace dominance across a broad spectrum of regional and littoral missions as well as open-ocean, "blue water" missions. The program design goal is to produce a submarine flexible enough to carry out seven very different missions:
Covert Strike by launching land-attack missiles from vertical launchers and torpedo tubes;
Anti-Submarine Warfare with an advanced combat system and a flexible payload of torpedoes;
Anti-Ship Warfare, again, using the advanced combat system and torpedoes;
Battle Group Support with advanced electronic sensors and communications equipment;
Covert Intelligence, Surveillance and Reconnaissance, using sensors to collect critical intelligence and locate radar sites, missile batteries and command sites as well as to monitor communications and track ship movements;
Covert Minelaying against enemy shipping; and
Special Operations, including search and rescue, reconnaissance, sabotage, diversionary attacks, and direction of fire support and strikes.
The New Attack Submarine is designed for multi-mission operations and enhanced operational flexibility. SEAWOLF (SSN-21)-Class quieting has been incorporated in a smaller hull while military performance has been maintained or improved. Compared with the Seawolf, the NSSN is slower, carries fewer weapons, and is less capable in diving depth and arctic operations. On the other hand, the NSSN is expected to be as quiet as the Seawolf, will incorporate a vertical launch system and have improved surveillance as well as special operations characteristics to enhance littoral warfare capability. While the 688-I submarines are noisier than the improved Russian Akula class, the Seawolf is quieter than Akula and the upcoming Russian SSN-P-IX class. The primary design driver for the NSSN is acoustic quietness equal to that of the Seawolf, even at the cost of reducing maximum top speed. With a focus on the littoral battlespace, the New Attack Submarine has improved magnetic stealth, sophisticated surveillance capabilities, and Special Warfare enhancements.

Operating in the shallow waters of littoral areas imposes a different accoustic environment for which previous submarinen classes were optimized. As reported in ONR Ocean Science and Engineering Newsletter # 2 (Feb. 1997) it is well known that as a result of the selective frequency effect of the shallow-water sound channel, a band of frequencies exist in which the propagation is enhanced (i.e., the transmission loss is relatively small). This "optimum frequency" regime arises from the combined effect of the volume attenuation at the higher frequencies and the loss due to interaction with the sea bottom at the lower frequencies.

Because of the proximity of the boundaries in shallow water, multipath transmission and multi-angle scattering from the sea bottom are concomitant characteristics of shallow-water acoustic reverberation. Consequently, long-range reverberation in shallow water is far more complex than the deep-water case. Because of interaction with the bottom, long range sound propagation in shallow water is characterized by separation of the constituent modes as a result of the differences in modal group velocities. This results in elongated, low amplitude signals. Further, because of the non-uniform effects of the interaction--e.g., the higher-angle modes suffer greater attenuation--only several modes may be needed to characterize the sound field. Hence, mode filtering is a useful approach for investigating multipath fields in shallow water.
The spatial structure of the accoustic signal in the waveguide formed by the surface and bottom in shallwo water is significantly different from the that in the free field of deep water. Hence, due to modal interference in a waveguide, conventional beamforming techniques cannot be used. Several on mode filtering methods are possible source ranging and depth estimation in the shallow water wave guide. Signals of several modes may be separated, and after correction for arrival time and phase, these filtered normal modes may be recombined to obtain a compressed and enhanced signal.

In some shallow water regions very strong and sharp summer thermoclines exist, and are accompanied by conspicuous internal waves. Anomalous attenuation of sound between 300 Hz and 1200 Hz is associated with these conditions, with very large variations (as much as 30 dB at some frequencies) in the frequency response of the transmission loss. These abnormally large attenuation can be attributed to internal wave-induced acoustic mode coupling. In particular, the internal waves cause a transfer of energy into the higher-order modes, which, since they interact more with the lossy bottom, leads to a frequency-dependent energy loss (or attenuation) in the sound wave.

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The New Attack Submarine is engineered for maximum design flexibility, responsiveness to changing missions and threats, and affordable insertion of new technologies to ensure that it will continue to be the right submarine well into the 21st Century. Integrated electronic systems with Commercial-Off-The-Shelf (COTS) components facilitate state-of-the-art technology introduction throughout the life of the class and avoid unit obsolescence. The Navy has never attempted such a large-scale integration effort on a submarine. While the BSY-1 and BSY-2 systems did have some level of integration, the NSSN combat system will have to be totally integrated. Both the BSY-1 combat system for the Improved Los Angeles-class and the BSY-2 combat system for the Seawolf-class submarines had problems that resulted in late delivery and increased costs.
The Command, Control, Communications, and Intelligence (C3I) electronics packages also promote maximum flexibility for growth and upgrade. Coupled with the Modular Isolated Deck Structure (MIDS) and open-system architecture, this approach results in a lower cost and effective, command and control structure for fire control, navigation, electronic warfare, and communications connectivity.

The New Attack Submarine's sonar system is state-of-the-art and has more processing power than today's entire submarine fleet combined to process and distribute data received from its spherical bow array, high-frequency array suite, dual towed arrays, and flank array suite.

The New Attack Submarine's sail configuration houses two new photonics masts for improved imaging functions, and improved electronics support measures mast, and multi-mission masts that cover the frequency domain for full-spectrum, high data-rate communications. The sail is also designed for future installation of a special mission-configurable mast for enhanced flexibility and warfighting performance.

The VIRGINIA Class submarine program has been designed with long-term technological innovation in mind. The built-in flexibility of VIRGINIA, including incorporation of modular design techniques, open architecture, and COTS components, allows for technological insertion and innovation. As an example of the flexibility inherent in the design of VIRGINIA, the Navy anticipates placing an advanced sail on hulls 5-6 of the VIRGINIA Class. The new sail shape and size might well provide the required volume for advanced future payloads.

The new attack submarine is armed with a variety of weapons. It carries the most advanced heavyweight torpedoes, mines, Tomahawk cruise missiles, and Unmanned Undersea Vehicles (UUVs) for horizontal launch. In addition, Tomahawk missiles are carried in vertical launch tubes. The New Attack Submarine also features an integral Lock-Out/Lock-In chamber for special operations and can host Special Operations Forces' underwater delivery vehicles.

Reducing acquisition and life-cycle costs is a major objective of the New Attack Submarine design and engineering process. Cost avoidance is anticipated through the application of concurrent engineering design/build teams, computer-aided design and electronic visualization tools, system simplification, parts standardization, and component elimination. These innovations are intended to ensure that the ship is affordable in sufficient numbers to satisfy America's future nuclear attack submarine force level requirements.

The New Attack Submarine Program Office is applying the lessons learrned from successful government and industry programs of similar scope and complexity to improve producibility and lower costs. Integrated Product and Process Development (IPPD) teams bring the combined experience of the shipbuilders, vendors, designers and engineers, and ship operators to bear on the ship design. The early involvement of production people on these teams is intended to provide a match between the design and the shipbuilder's construction processes and facilites, a smoother transition from design to production, and reduction in the number of changes during construction. The ship is designed using a state-of-the-art digital database, which allows members of the IPPD teams to work from a single design database and provides three-dimensional electronic mockups throughout the design process.

The Milestone I COEA examined twelve alternatives. The JROC reviewed and validated the key performance parameters (KPPs) for the selected new attack submarine design. The Milestone I DAB approved NSSN to enter Phase I in August 1994. The Milestone II DAB approved NSSN to enter the Demonstration and Validation Phase on June 30, 1995.

A number of systems that will be part of NSSN underwent testing in FY97. TB-29 towed array and the ADCAP Torpedo Block Upgrade III completed OPEVAL in September 1997. The Submarine Advanced Tomahawk Weapons Control System (Sub-ATWCS), Ring-laser Gyro Navigator and Doppler Sonar Velocity Log underwent operational testing as well. A scale model of the propulsor was tested. When USS SEAWOLF went to sea, the following equipment common or similar to NSSN were observed; propulsor, wide aperture array (WAA), impressed current cathodic protection system, and active shaft grounding system.